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Sulloway (1996, 2011) proposed that the adaptive problems imposed by parents on children will create different “niches” for children, depending on their birth order. Specifically, because parents often favor the oldest child, the firstborn tends to be relatively more conservative and more likely to support the status quo. Second-borns, however, have little to gain by supporting the existing structure and everything to gain by rebelling against it. Later-borns, especially middle-borns, according to Sulloway, develop a more rebellious personality because they have the least to gain by maintaining the existing order; studies of birth order and personality confirm this prediction (Healey & Ellis, 2007). The youngest, on the other hand, might receive more parental investment than middle children, as parents often let out all the stops to invest in their final direct reproductive vehicle. Salmon and Daly (1998) find support for these predictions. They discovered that middle-borns differ from first- and last-borns in scoring lower on measures of family solidarity and identity. Middle-borns, for example, are less likely to name a genetic relative as the person to whom they feel closest. They are also less likely to assume the role of family genealogist. Middle-borns, compared to firstborns and last-borns, are less positive in attitudes toward their families and less likely to help a family member who needs help (Salmon, 2003). These and other results (Salmon, 1999) lend some support to Sulloway’s theory that birth order affects the niches a person selects. Firstborns are more likely to feel solidarity with parents and perceive them as dependable, whereas middle-borns appear more likely to invest in bonds outside of the family. Interestingly, middle-born children might receive less total investment from parents even if parents treat all their children equally (Hertwig, Davis, & Sulloway, 2002). This result occurs because firstborns receive all of their parents’ investments early in life before other children are born and last-borns receive all of their parents’ investments after all other children leave the house. Middle-borns, in contrast, must share their parents’ investments, because there is rarely a time when other siblings are not around. Even when parents strive to invest equally in their children, middle-borns end up on the short end of the stick—perhaps accounting for why middle-borns are less identified with their families (Hertwig et al., 2002).
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David M. Buss (Evolutionary Psychology: The New Science of the Mind)